This invention relates to sensor systems that are heated and/or cooled during service, and, more particularly, to a structure that reduces thermal expansion strains between the elements of the sensor assembly.
A sensor assembly receives radiated energy from a scene, and converts that energy into electrical signals for display and/or analysis. Many infrared and other types of sensor assemblies operate most efficiently and reliably when cooled to a cryogenic temperature, such as about the boiling point of liquid nitrogen, 77K, and are operated in a vacuum to provide thermal insulation for the sensor assembly and to avoid condensation of gases such as water vapor on the sensor assembly. To effect these conditions in service, the sensor assembly is mounted on the platform supported on a hollow tube termed a xe2x80x9ccold fingerxe2x80x9d, and within an evacuated dewar/vacuum enclosure. The dewar/vacuum enclosure typically includes an insulated vacuum housing having a window through which the sensor views an external scene.
When the sensor is to be used in service, the opposite end of the cold finger is cooled by a cooling device, causing heat to flow out of the cold finger, the platform, and the sensor assembly. After the sensor assembly reaches the required reduced operating temperature, it becomes operational.
In one design, the sensor assembly includes a planar radiation detector and a planar readout circuit joined to the detector in a face-to-face fashion by a plurality of bump interconnects, with the remaining space typically filled by an adhesive such as an epoxy. The radiation detector and the readout circuit are formed of different materials, with different coefficients of thermal expansion. The bumps provide both electrical communication between the detector and the readout circuit, and structural joining. This structure is well known in the art.
The sensor assembly is usually assembled and joined to the platform of the cold finger at about room temperature (300xc2x0 K.), with negligible strains between the radiation detector and the readout circuit. When the sensor assembly is cooled to the service temperature, differential thermal expansion strains between the radiation detector and the readout circuit arise because of the different coefficients of thermal expansion of the detector and the readout circuit. These differential thermal strains apply stresses to the bump interconnects, which can cause the bump interconnects, and thence the sensor assembly, to fail.
Several approaches have been suggested to overcome this problem. In one, taller bump interconnects are used to allow the bumps to deform, thereby reducing the incidence of failure. This approach is operable for sensor assemblies of small lateral extent, but is of limited usefulness for larger sensor assemblies. In another approach, the epoxy adhesive is omitted, but the resulting structure may have insufficient mechanical strength. In a third approach, different materials have been used in the sensor assembly in an attempt to reduce the thermal expansion mismatch, but the resulting sensor has a reduced operating performance. In a fourth approach, a readout circuit substrate is built into the sensor assembly between the readout circuit and the platform, to alter the thermal expansion properties of the readout circuit so as to be closer to those of the radiation detector. This approach is operable, but the readout circuit substrate also serves as a substantial thermal impedance between the sensor assembly and the cooler, increasing the cooldown time and thence the waiting time before operation of the sensor system can commence. This increased waiting time is unacceptable for some applications. The readout circuit substrate also changes the axial position of the image plane, resulting in physical incompatibility of systems that use the readout circuit substrate and those which do not. The use of the readout circuit substrate also increases the required lengths of wirebonds, making them more susceptible to shock and vibration. The use of the readout circuit substrate therefore may not be recommended for some applications.
Accordingly, there is a need for an approach to reducing the differential thermal expansion strains between the detector and the readout circuit of a sensor assembly which is to be cooled during operation. The present invention fulfills this need, and further provides related advantages.
The present invention provides a sensor/support structure having reduced differential thermal expansion strains between the detector and the readout circuit of the sensor assembly, and consequently a reduced likelihood of failure and increased service life. The performance of the sensor assembly is maintained at a high level. The present approach does not require any change in the sensor or other materials of construction, the position of the image plane, the wirebond lengths, or the bump heights, with the result that the present approach may be used with sensor assemblies whose structure is already optimized for sensor performance. The cooldown time of the sensor/support structure is not substantially increased as compared with a conventional structure, so that the waiting time between the start of cooldown and the start of operation of the sensor is not substantially lengthened. The present approach is operable in conjunction with sensor/support structures that must be heated and cooled repeatedly, and yields particularly favorable results in such applications.
In accordance with the invention, a sensor/support system comprises a sensor assembly comprising a radiation detector, and a readout circuit joined to the detector, preferably by a plurality of rigid electrical bump interconnects extending between the detector and the readout circuit. The sensor/support system further includes a support structure comprising a platform having a first side to which the sensor assembly is affixed and a second side oppositely disposed from the first side.
A stabilization structure is affixed to the second side of the platform. The stabilization structure is preferably a shim affixed to the second side of the platform remote from the sensor assembly. The stabilization structure reduces the strain in the interconnect when the temperature of the sensor/support system is changed, as compared with the strain in the interconnect when the temperature is changed in the absence of the stabilization structure.
The shim stabilization structure is applied to the side of the platform remote from the sensor assembly, which is a non-vacuum space for most dewars. The shim is not positioned between the sensor assembly and the platform, so that it does not act as a thermal impedance to reduce the cooling rate and lengthen the waiting time to commence operation of the sensor assembly, as is the case for systems utilizing an expansion controlled readout circuit substrate. The structure, materials of construction, and geometry of the sensor assembly are not altered when the present invention is used, so that the sensor assembly may be separately optimized for sensor optoelectronic performance.